Most dock levelers or dock boards are installed in a pit formed in a dock. The leveler includes a ramp or deck hinged at its rear edge to a frame and an extension lip hinged to the front edge of the ramp. Due to the weight of the ramp, a spring-urged life arm is employed to bias the ramp upwardly to an inclined position, while a releasable hold-down mechanism normally connects the frame with the ramp and thereby holds the ramp against upward movement. When not in use, the ramp is held in a cross-traffic position in which the ramp is flush with the upper surface of the dock. Before a truck or other carrier is moved up to the dock, the operator manually releases the hold-down mechanism so that the lift arm pivots the ramp upwardly to an inclined position. As a consequence of the upward pivotal movement of the ramp, a mechanism is actuated to pivot the ramp lip upwardly and outwardly to an extended position. The operator then walks outwardly on the inclined ramp, and his weight overcomes the upward force of the lift arm to lower the ramp and move the lip into engagement with the truck bed. This conventional hold-down mechanism is arranged so that the ramp can be moved downwardly without restraint, yet engagement of the hold-down mechanism prevents upward movement of the ramp or, in the alternative, permits upward movement of the ramp through only a limited extent as permitted by a compression spring associated with the hold-down mechanism.
While many hold-down mechanisms are known which operate generally in the above described manner, nevertheless these known structures have generally possessed structural and/or operational disadvantages which have resulted in these structures performing in less than an optimum manner. For example, in some of the known mechanisms, while the hold-down mechanism may perform in a highly desirable manner when subjected to certain types of loads or when in specific positions, nevertheless these mechanisms do not result in optimum performance when subjected to loads of other magnitudes or when disposed in other operational position.
Accordingly, it is an object of the present invention to provide an improved hold-down mechanism for a dock leveler, which hold-down mechanism permits the ramp of the leveler to raise when the truck bed raises, as when a load is removed therefrom, which improved hold-down mechanism provides for optimum movement and control of the ramp under the loading and positional requirements experienced by the dock leveler during normal operation thereof.
A further object of the present invention is to provide an improved hold-down mechanism, as aforesaid, which utilizes a resilient device which causes a large hold-down force to be imposed on the ramp when disposed within its working range and engaged with a truck bed, and which decreases the hold-down force when the ramp is lifted upwardly by the truck bed to simplify lifting of the ramp.
Still a further object of the invention is to provide an improved hold-down mechanism, as aforesaid, which utilizes a torsion spring associated with a crank arm, the latter being interconnected through an extendable unidirectional clamping structure to the ramp.
Another object of the invention is to provide an improved hold-down mechanism, as aforesaid, which results in the extendable clamping structure and the crank moving through an over-the-center position when the ramp is moved into its raised uppermost position, whereby the crank is urged against a stop which prevents yielding of the torsion spring.
Still another object of this invention is to provide an improved hold-down mechanism, as aforesaid, which is relatively inexpensive to fabricate, durable and reliable in operation and effective in maintaining the ramp in selected positions including a substantially horizontal cross-traffic position.